Neural stimulator system

ABSTRACT

An implantable neural stimulator method for modulating excitable tissue in a patient including: implanting a neural stimulator within the body of the patient such that one or more electrodes of the neural stimulator are positioned at a target site adjacent to or near excitable tissue; generating an input signal with a controller module located outside of, and spaced away from, the patient&#39;s body; transmitting the input signal to the neural stimulator through electrical radiative coupling; converting the input signal to electrical pulses within the neural stimulator; and applying the electrical pulses to the excitable tissue sufficient to modulate said excitable tissue.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 14/068,750, filed Oct. 31, 2013, now U.S. Pat. No.9,566,449, which is a divisional application of U.S. patent application Ser. No. 13/551,050 filed Jul. 17, 2012, now U.S. Pat. No. 9,409,030, issued Aug. 9, 2016, which is continuation of International Application No. PCT/US2012/023029, filed Jan. 27, 2012, which claims the benefit of priority to U.S. provisional Patent Application No. 61/437,561, filed Jan. 28, 2011, all of which applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

This description is related to implanted neural stimulators.

BACKGROUND

Neural modulation of neural tissue in the body by electrical stimulation has become an important type of therapy for chronic disabling conditions, such as chronic pain, problems of movement initiation and control, involuntary movements, dystonia, urinary and fecal incontinence, sexual difficulties, vascular insufficiency, heart arrhythmia and more. Electrical stimulation of the spinal column and nerve bundles leaving the spinal cord was the first approved neural modulation therapy and been used commercially since the 1970s. Implanted electrodes are used to pass pulsatile electrical currents of controllable frequency, pulse width and amplitudes. Two or more electrodes are in contact with neural elements, chiefly axons, and can selectively activate varying diameters of axons, with positive therapeutic benefits. A variety of therapeutic intra-body electrical stimulation techniques are utilized to treat neuropathic conditions that utilize an implanted neural stimulator in the spinal column or surrounding areas, including the dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers and peripheral nerve bundles leaving the dorsal column or brain, such as vagus-, occipital-, trigeminal, hypoglossal-, sacral-, and coccygeal nerves.

SUMMARY

In one aspect, an implantable neural stimulator includes one or more electrodes, a first antenna, and one or more circuits. The one or more electrodes configured to apply one or more electrical pulses to neural tissue. The first antenna is a dipole antenna and is configured to receive, from a second antenna through electrical radiative coupling, an input signal containing electrical energy, the second antenna being physically separate from the implantable neural stimulator; and transmit, to the second antenna through electrical radiative coupling, one or more feedback signals. The one or more circuits are connected to the dipole antenna and configured to create one or more electrical pulses suitable for stimulation of neural tissue using the electrical energy contained in the input signal; supply the one or more electrical pulses to the one or more electrodes such that the one or more electrodes apply the one or more electrical pulses to neural tissue; generate a stimulus feedback signal, the stimulus feedback signal indicating one or more parameters of the one or more electrical pulses applied to the neural tissue by the one or more electrodes; and send the stimulus feedback signal to the dipole antenna such that the dipole antenna transmits the stimulus feedback signal to the second antenna through electrical radiative coupling.

Implementations of this and other aspects may include the following features. The input signal may also contain information encoding stimulus parameters for the one or more electrical pulses and the one or more circuits are configured to create the electrical pulses based on the information encoding stimulus parameters. The one or more parameters may include an amplitude of the one or more electrical pulses or an impedance of the one or more electrodes. The one or more circuits may be configured such that a level of the input signal directly determines an amplitude of the one or more electrical pulses applied to the neural tissue by the one or more electrodes.

The one or more circuits may be configured to limit a characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes so that a charge per phase resulting from the one or more electrical pulses remains below a threshold level; generate a limit feedback signal when the charge per phase resulting from the one or more electrical pulses would have exceeded the threshold level if the one or more circuits had not limited the characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes so that the charge per phase resulting from the one or more electrical pulses remained below the threshold level; and send the limit feedback signal to the dipole antenna such that the dipole antenna transmits the limit feedback signal to the second antenna through electrical radiative coupling. The characteristic of the one or more pulses applied to the neural tissue by the one or more electrodes may be a current level and the threshold level may be a current threshold level.

The one or more circuits may be configured to create the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge. To create the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge, the one or more circuits may include at least one capacitor in series with the one or more electrodes.

The one or more circuits may include a waveform conditioning component to create the one or more electrical pulses suitable for stimulation of neural tissue using the electrical energy contained in the input signal; an electrode interface connected to the waveform conditioning circuit, the electrode interface being configured to receive the one or more electrical pulses from the waveform condition circuit and supply the one or more electrical pulses to the one or more electrodes; and a controller connected to the electrode interface, the controller being configured to generate the stimulus feedback signal and send the stimulus feedback signal to the dipole antenna. The waveform conditioning component may include a rectifier connected to the dipole antenna, the rectifier configured to receive the input signal from the dipole antenna and generate a rectified electrical waveform based on the input signal; a charge balance component configured to create the one or more electrical pulses based on the rectified electrical waveform such that the one or more electrical pulses result in a substantially zero net charge at the one or more electrodes; and a charge limiter configured to limit a characteristic of the one or more electrical pulses so that a charge per phase resulting from the one or more electrical pulses remains below a threshold level, wherein the limited electrical pulses are sent to the electrode interface from the charge limiter.

The one or more electrodes may include a plurality of electrodes and the one or more circuits may be configured to selectively designate each of the electrodes to act as a stimulating electrode, act as a return electrode, or be inactive.

The electrodes, the dipole antenna, and one or more circuits may be configured and geometrically arranged to be located at one of the following locations: epidural space of the spinal column, near, beneath or on the dura mater of the spinal column, in tissue in close proximity to the spinal column, in tissue located near the dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers and/or peripheral nerve bundles leaving the dorsal column of the spine, abdominal, thoracic, and trigeminal ganglia, peripheral nerves, deep brain structures, cortical surface of the brain and sensory or motor nerves.

The implantable neural stimulator may not include an internal power source. The one or more circuits may include only passive components. The input signal may have a carrier frequency in the range from about 300 MHz to about 8 GHz

In another aspect, a system includes a controller module. The controller module includes a first antenna and one or more circuits. The first antenna is configured to send an input signal containing electrical energy to a second antenna through electrical radiative coupling. The second antenna is a dipole antenna and is located in an implantable neural stimulator that is configured to create one or more electrical pulses suitable for stimulation of neural tissue using the input signal, wherein the implantable neural stimulator is separate from the controller module. The first antenna is also configured to receive one or more signals from the dipole antenna. The one or more circuits are configured to generate the input signal and send the input signal to the dipole antenna; extract a stimulus feedback signal from one or more signals received by the first antenna, the stimulus feedback signal being sent by the implantable neural stimulator and indicating one or more parameters of the one or more electrical pulses; and adjust parameters of the input signal based on the stimulus feedback signal.

Implementations of this and other aspects may include one or more of the following features. For example, the one or more parameters of the electrical pulses may include an amplitude of the one or more electrical pulses as applied to the neural tissue and the one or more circuits are configured to adjust a power of the input signal based on the amplitude of the one or more electrical pulses. The one or more circuits may be configured to obtain a forward power signal that is reflective of an amplitude of a signal sent to the first antenna; obtain a reverse power signal that is reflective of an amplitude of a reflected portion of the signal sent to the first antenna; determine a mismatch value indicative of a magnitude of an impedance mismatch based on the forward power signal and the reverse power signal; and adjust parameters of the input signal based on the mismatch value.

The system may include the implantable neural stimulator. The implantable neural stimulator may include one or more electrodes configured to apply the one or more electrical pulses to neural tissue and one or more circuits. The one or more circuits may be configured to create the one or more electrical pulses; supply the one or more electrical pulses to the one or more electrodes such that the one or more electrodes apply the one or more electrical pulses to neural tissue; generate the stimulus feedback signal; and send the stimulus feedback signal to the dipole antenna such that the dipole antenna transmits the stimulus feedback signal to the first antenna through electrical radiative coupling.

The input signal may also contain information encoding stimulus parameters for the one or more electrical pulses and the implantable neural stimulator is configured to create the one or more electrical pulses based on the information encoding stimulus parameters. The one or more parameters of the one or more electrical pulses may include an amplitude of the one or more electrical pulses or an impedance of the one or more electrodes. The one or more circuits of the implantable neural stimulator may be configured such that a level of the input signal directly determines an amplitude of the one or more electrical pulses applied to the neural tissue by the one or more electrodes.

The one or more circuits of the implantable neural stimulator may be configured to limit a characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes so that a charge per phase resulting from the one or more electrical pulses remain below a threshold level; generate a limit feedback signal when the charge per phase resulting from the one or more electrical pulses would have exceeded the threshold level if the one or more circuits had not limited the characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes so that the charge per phase resulting from the one or more electrical pulses remained below the threshold level; and send the limit feedback signal to the dipole antenna such that the dipole antenna transmits the limit feedback signal to the second antenna through electrical radiative coupling. The characteristic of the one or more pulses applied to the neural tissue by the one or more electrodes may be a current level and the threshold level may be a current threshold level. The one or more circuits of the controller module may be configured to receive the limit feedback signal from the dipole antenna; and attenuate the input signal in response to receiving the limit feedback signal.

The one or more circuits may be configured to create the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge. To create the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge, the one or more circuits of the implantable neural stimulator may include at least one capacitor in series with the one or more electrodes.

The one or more circuits of the implantable neural stimulator may include a waveform conditioning component to create the one or more electrical pulses suitable for stimulation of neural tissue using electrical energy contained in the input signal; an electrode interface connected to the waveform conditioning circuit, the electrode interface being configured to receive the one or more electrical pulses from the waveform condition circuit and supply the one or more electrical pulses to the one or more electrodes; and a controller connected to the electrode interface, the controller being configured to generate the stimulus feedback signal and send the stimulus feedback signal to the dipole antenna. The waveform conditioning component may include a rectifier connected to the dipole antenna, the rectifier configured to receive the input signal from the dipole antenna and generate a rectified electrical waveform based on the input signal; a charge balance component configured to create the one or more electrical pulses based on the rectified electrical waveform such that the one or more electrical pulses result in a substantially zero net charge at the one or more electrodes; and a charge limiter configured to limit the a characteristic of the one or more electrical pulses so that a charge per phase resulting from the one or more electrical pulses remains below a threshold level, wherein the limited electrical pulses are sent to the electrode interface through the charge limiter.

The implantable neural stimulator may include a plurality of electrodes. The one or more circuits of the controller module may be configured to generate a control signal that designates which electrodes act as stimulating electrodes, which electrodes act as return electrodes, and which electrodes are inactive; and send the control signal to the first antenna such that the first antenna transmits the control signal to the dipole antenna through electrical radiative coupling. The one or more circuits of the implantable neural stimulator may be configured to selectively designate each of the electrodes to act as a stimulating electrode, act as a return electrode, or be inactive based on the control signal.

The implantable neural stimulator may not include an internal power source. The one or more circuits of the implantable neural stimulator may include only passive components. The input signal has a carrier frequency in the range from about 300 MHz to about 8 GHz

In another aspect, a method includes implanting a neural stimulator within a patient's body such that one or more electrodes of the neural stimulator are positioned to apply electrical pulses to neural tissue. The neural stimulator includes a first antenna configured to receive an input signal containing electrical energy. The first antenna is a dipole antenna. The neural stimulator is configured to create one or more electrical pulses suitable for stimulation of the neural tissue using the electrical energy contained in the input signal; supply the one or more electrical pulses to the one or more electrodes such that the one or more electrodes apply the one or more electrical pulses to the neural tissue; generate a stimulus feedback signal, the stimulus feedback signal indicating one or more parameters of the one or more electrical pulses applied to the neural tissue by the one or more electrodes; and transmit the stimulus feedback signal from the dipole antenna to a second antenna through electrical radiative coupling. The method also includes positioning a controller module in proximity to the patient's body, wherein the controller module is connected to the second antenna; and operating the controller module such that the controller module generates the input signal and sends the input signal to the second antenna such that second antenna transmits the input signal to the dipole antenna within the implanted neutral stimulator through electrical radiative coupling; extracts the stimulus feedback signal from one or more signals received by the second antenna; and adjusts parameters of the input signal based on the stimulus feedback signal.

Implementations of this and other aspects may include one or more of the following features. For example, the parameters may include an amplitude of the one or more electrical pulses or an impedance of the one or more electrodes. The neural stimulator may be configured to create the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge within the patient's body. The neural stimulator may be configured to selectively designate one or more electrodes to act as a stimulating electrode, act as a return electrode, or be inactive.

Implanting the neural stimulator may include implanting the neural stimulator at one of the following locations within the patient's body: epidural space of the spinal column, near, beneath or on the dura mater of the spinal column, in tissue in close proximity to the spinal column, in tissue located near the dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers and/or peripheral nerve bundles leaving the dorsal column of the spine, abdominal, thoracic, and trigeminal ganglia, peripheral nerves, deep brain structures, cortical surface of the brain and sensory or motor nerves.

The implanted neural stimulator may not include an internal power source. The implanted neutral stimulator may include at least one capacitor in series with the one or more electrodes.

Implementations of the technology described herein may include one or more of the following advantages. For example, implementations may avoid the numerous failure modes associated with implanted pulse generator modules that are connected to electrodes through physical leads, such as loss of electrical continuity due to mechanical flexure, mechanical dislodgement caused by natural motion of the body, impingement of the lead electrode assembly into tissue, infection, and uncomfortable irritation.

Various implementations may be useful for neural modulation therapies involving the brain. Areas of the brain can be stimulated to help treat the symptoms of chronic pain, assist with movement disorders, clinical depression, control epilepsy and more. The cortex of the brain is a neural stimulation target where stimulating electrodes are positioned outside the dura mater. Various implementations may employ lead/electrode volume more than ten times less than electrodes currently being used for such stimulation. Such electrodes may require creation of a large hole in the skull, 1.0 sq mm or more in diameter. Some implementations can be ejected from an extremely small injector lumen, such as a typical 22-gauge needle used in laparoscopic or endoscopic placements. Thus, some implementations may employ a hole in the skull much smaller than current devices. If several stimulators are to be inserted, a catheter can be placed through the hole, steered with a removable stylet, and the stimulators can be pushed out of the catheter placed at their respective locations.

Deep brain stimulation (DBS) is used to treat the symptoms arising from chronic pain, movement disorders, obsessive-compulsive disorders, and epilepsy. Target locations for electrode placement to treat chronic pain symptoms with DBS include the sensory thalamus and periventricular gray matter. Target locations in the brain for treatment of the symptoms of movement disorders, such as Parkinson' include ventral intermediate thalamus, subthalamic nucleus, and the globus pallidus. The hypothalamus is one target location for electrode placement to treat epileptic symptoms with DBS. Placement of various implementations deep in the brain may cause minimal acute trauma or chronic reactions due to the small size of the stimulator.

Applications of the technology near the spinal cord may include advantages of ease of insertion, elimination of extension wires, and no requirement for an implantable pulse generator to administer a chronic therapy. Spinal cord stimulation is used to treat chronic neuropathic pain, especially low back pain and radiculopathy, vascular insufficiency in the feet or hands, angina, and more. Various implementations of the technology may allows placement of electrodes in the epidural space, between the dura mater and arachnoid membranes, which is standard practice in the art, or subdurally in the intrathecal space, since significant reactions and scarring would be minimal. Insertion in any of these spaces may be done by ejecting the device from a 22-gauge needle or out of a catheter steered to the proper position by a removable stylet. In some implementations, once in position, no further skin incisions or placement of extensions, receivers or implanted pulse generators are needed. Various implementations of the wireless neural modulation system may have significant advantages due to the small size and lack of extension wires for transfer of energy, allowing placement with minimal trauma and long term effective therapy in places where larger implantable devices could cause more scar tissue and tissue reactions that may affect efficacy and safety.

Various implementations may be inherently low in cost compared to existing implantable neural modulation systems, and this may lead to wider adoption of neural modulation therapy for patients in need as well as reduction in overall cost to the healthcare system.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts a high-level diagram of an example of a wireless neural stimulation system.

FIG. 2 depicts a detailed diagram of an example of the wireless neural stimulation system.

FIG. 3 is a flowchart showing an example of the operation of the wireless neural stimulator system.

FIG. 4 depicts a flow chart showing an example of the operation of the system when the current level at the electrodes is above the threshold limit.

FIG. 5 is a diagram showing examples of signals that may be used to detect an impedance mismatch.

FIG. 6 is a diagram showing examples of signals that may be employed during operation of the wireless neural stimulator system.

FIG. 7 is a flow chart showing a process for the user to control the implantable wireless neural stimulator through an external programmer in an open loop feedback system.

FIG. 8 is another example flow chart of a process for the user to control the wireless stimulator with limitations on the lower and upper limits of current amplitude.

FIG. 9 is yet another example flow chart of a process for the user to control the wireless neural stimulator through preprogrammed parameter settings.

FIG. 10 is still another example flow chart of a process for a low battery state for the RF pulse generator module.

FIG. 11 is yet another example flow chart of a process for a Manufacturer's Representative to program the implanted wireless neural stimulator.

FIG. 12 is a circuit diagram showing an example of a wireless neural stimulator.

FIG. 13 is a circuit diagram of another example of a wireless neural stimulator.

DETAILED DESCRIPTION

In various implementations, a neural stimulation system may be used to send electrical stimulation to targeted nerve tissue by using remote radio frequency (RF) energy with neither cables nor inductive coupling to power the passive implanted stimulator. The targeted nerve tissues may be, for example, in the spinal column including the spinothalamic tracts, dorsal horn, dorsal root ganglia, dorsal roots, dorsal column fibers, and peripheral nerves bundles leaving the dorsal column or brainstem, as well as any cranial nerves, abdominal, thoracic, or trigeminal ganglia nerves, nerve bundles of the cerebral cortex, deep brain and any sensory or motor nerves.

For instance, in some implementations, the neural stimulation system may include a controller module, such as an RF pulse generator module, and a passive implanted neural stimulator that contains one or more dipole antennas, one or more circuits, and one or more electrodes in contact with or in proximity to targeted neural tissue to facilitate stimulation. The RF pulse generator module may include an antenna and may be configured to transfer energy from the module antenna to the implanted antennas. The one or more circuits of the implanted neural stimulator may be configured to generate electrical pulses suitable for neural stimulation using the transferred energy and to supply the electrical pulses to the electrodes so that the pulses are applied to the neural tissue. For instance, the one or more circuits may include wave conditioning circuitry that rectifies the received RF signal (for example, using a diode rectifier), transforms the RF energy to a low frequency signal suitable for the stimulation of neural tissue, and presents the resulting waveform to an electrode array. The one or more circuits of the implanted neural stimulator may also include circuitry for communicating information back to the RF pulse generator module to facilitate a feedback control mechanism for stimulation parameter control. For example, the implanted neural stimulator may send to the RF pulse generator module a stimulus feedback signal that is indicative of parameters of the electrical pulses, and the RF pulse generator module may employ the stimulus feedback signal to adjust parameters of the signal sent to the neural stimulator.

FIG. 1 depicts a high-level diagram of an example of a neural stimulation system. The neural stimulation system may include four major components, namely, a programmer module 102, a RF pulse generator module 106, a transmit (TX) antenna 110 (for example, a patch antenna, slot antenna, or a dipole antenna), and an implanted wireless neural stimulator 114. The programmer module 102 may be a computer device, such as a smart phone, running a software application that supports a wireless connection 114, such as Bluetooth®. The application can enable the user to view the system status and diagnostics, change various parameters, increase/decrease the desired stimulus amplitude of the electrode pulses, and adjust feedback sensitivity of the RF pulse generator module 106, among other functions.

The RF pulse generator module 106 may include communication electronics that support the wireless connection 104, the stimulation circuitry, and the battery to power the generator electronics. In some implementations, the RF pulse generator module 106 includes the TX antenna embedded into its packaging form factor while, in other implementations, the TX antenna is connected to the RF pulse generator module 106 through a wired connection 108 or a wireless connection (not shown). The TX antenna 110 may be coupled directly to tissue to create an electric field that powers the implanted neural stimulator module 114. The TX antenna 110 communicates with the implanted neural stimulator module 114 through an RF interface. For instance, the TX antenna 110 radiates an RF transmission signal that is modulated and encoded by the RF pulse generator module 110. The implanted wireless neural stimulator module 114 contains one or more antennas, such as dipole antenna(s), to receive and transmit through RF interface 112. In particular, the coupling mechanism between antenna 110 and the one or more antennas on the implanted neural stimulation module 114 is electrical radiative coupling and not inductive coupling. In other words, the coupling is through an electric field rather than a magnetic field.

Through this electrical radiative coupling, the TX antenna 110 can provide an input signal to the implanted neural stimulation module 114. This input signal contains energy and may contain information encoding stimulus waveforms to be applied at the electrodes of the implanted neural stimulator module 114. In some implementations, the power level of this input signal directly determines an applied amplitude (for example, power, current, or voltage) of the one or more electrical pulses created using the electrical energy contained in the input signal. Within the implanted wireless neural stimulator 114 are components for demodulating the RF transmission signal, and electrodes to deliver the stimulation to surrounding neuronal tissue.

The RF pulse generator module 106 can be implanted subcutaneously, or it can be worn external to the body. When external to the body, the RF generator module 106 can be incorporated into a belt or harness design to allow for electric radiative coupling through the skin and underlying tissue to transfer power and/or control parameters to the implanted neural stimulator module 114, which can be a passive stimulator. In either event, receiver circuit(s) internal to the neural stimulator module 114 can capture the energy radiated by the TX antenna 110 and convert this energy to an electrical waveform. The receiver circuit(s) may further modify the waveform to create an electrical pulse suitable for the stimulation of neural tissue, and this pulse may be delivered to the tissue via electrode pads.

In some implementations, the RF pulse generator module 106 can remotely control the stimulus parameters (that is, the parameters of the electrical pulses applied to the neural tissue) and monitor feedback from the wireless neural stimulator module 114 based on RF signals received from the implanted wireless neural stimulator module 114. A feedback detection algorithm implemented by the RF pulse generator module 106 can monitor data sent wirelessly from the implanted wireless neural stimulator module 114, including information about the energy that the implanted wireless neural stimulator module 114 is receiving from the RF pulse generator and information about the stimulus waveform being delivered to the electrode pads. In order to provide an effective therapy for a given medical condition, the system can be tuned to provide the optimal amount of excitation or inhibition to the nerve fibers by electrical stimulation. A closed loop feedback control method can be used in which the output signals from the implanted wireless neural stimulator module 114 are monitored and used to determine the appropriate level of neural stimulation current for maintaining effective neuronal activation, or, in some cases, the patient can manually adjust the output signals in an open loop control method.

FIG. 2 depicts a detailed diagram of an example of the neural stimulation system. As depicted, the programming module 102 may comprise user input system 202 and communication subsystem 208. The user input system 221 may allow various parameter settings to be adjusted (in some cases, in an open loop fashion) by the user in the form of instruction sets. The communication subsystem 208 may transmit these instruction sets (and other information) via the wireless connection 104, such as Bluetooth or Wi-Fi, to the RF pulse generator module 106, as well as receive data from module 106.

For instance, the programmer module 102, which can be utilized for multiple users, such as a patient's control unit or clinician's programmer unit, can be used to send stimulation parameters to the RF pulse generator module 106. The stimulation parameters that can be controlled may include pulse amplitude, pulse frequency, and pulse width in the ranges shown in Table 1. In this context the term pulse refers to the phase of the waveform that directly produces stimulation of the tissue; the parameters of the charge-balancing phase (described below) can similarly be controlled. The patient and/or the clinician can also optionally control overall duration and pattern of treatment.

Stimulation Parameter Table 1 Pulse Amplitude:  0 to 20 mA Pulse Frequency: 0 to 2000 Hz Pulse Width:   0 to 2 ms

The implantable neural stimulator module 114 or RF pulse generator module 114 may be initially programmed to meet the specific parameter settings for each individual patient during the initial implantation procedure. Because medical conditions or the body itself can change over time, the ability to re-adjust the parameter settings may be beneficial to ensure ongoing efficacy of the neural modulation therapy.

The programmer module 102 may be functionally a smart device and associated application. The smart device hardware may include a CPU 206 and be used as a vehicle to handle touchscreen input on a graphical user interface (GUI) 204, for processing and storing data.

The RF pulse generator module 106 may be connected via wired connection 108 to an external TX antenna 110. Alternatively, both the antenna and the RF pulse generator are located subcutaneously (not shown).

The signals sent by RF pulse generator module 106 to the implanted stimulator 114 may include both power and parameter-setting attributes in regards to stimulus waveform, amplitude, pulse width, and frequency. The RF pulse generator module 106 can also function as a wireless receiving unit that receives feedback signals from the implanted stimulator module 114. To that end, the RF pulse generator module 106 may contain microelectronics or other circuitry to handle the generation of the signals transmitted to the stimulator module 114 as well as handle feedback signals, such as those from the stimulator module 114. For example, the RF pulse generator module 106 may comprise controller subsystem 214, high-frequency oscillator 218, RF amplifier 216, a RF switch, and a feedback subsystem 212.

The controller subsystem 214 may include a CPU 230 to handle data processing, a memory subsystem 228 such as a local memory, communication subsystem 234 to communicate with programmer module 102 (including receiving stimulation parameters from programmer module), pulse generator circuitry 236, and digital/analog (D/A) converters 232.

The controller subsystem 214 may be used by the patient and/or the clinician to control the stimulation parameter settings (for example, by controlling the parameters of the signal sent from RF pulse generator module 106 to neural stimulator module 114). These parameter settings can affect, for example, the power, current level, or shape of the one or more electrical pulses. The programming of the stimulation parameters can be performed using the programming module 102, as described above, to set the repetition rate, pulse width, amplitude, and waveform that will be transmitted by RF energy to the receive (RX) antenna 238, typically a dipole antenna (although other types may be used), in the wireless implanted neural stimulator module 214. The clinician may have the option of locking and/or hiding certain settings within the programmer interface, thus limiting the patient's ability to view or adjust certain parameters because adjustment of certain parameters may require detailed medical knowledge of neurophysiology, neuroanatomy, protocols for neural modulation, and safety limits of electrical stimulation.

The controller subsystem 214 may store received parameter settings in the local memory subsystem 228, until the parameter settings are modified by new input data received from the programming module 102. The CPU 206 may use the parameters stored in the local memory to control the pulse generator circuitry 236 to generate a stimulus waveform that is modulated by a high frequency oscillator 218 in the range from 300 MHz to 8 GHz. The resulting RF signal may then be amplified by RF amplifier 226 and then sent through an RF switch 223 to the TX antenna 110 to reach through depths of tissue to the RX antenna 238.

In some implementations, the RF signal sent by TX antenna 110 may simply be a power transmission signal used by stimulator module 114 to generate electric pulses. In other implementations, a telemetry signal may also be transmitted to the stimulator module 114 to send instructions about the various operations of the stimulator module 114. The telemetry signal may be sent by the modulation of the carrier signal (through the skin if external, or through other body tissues if the pulse generator module 106 is implanted subcutaneously). The telemetry signal is used to modulate the carrier signal (a high frequency signal) that is coupled onto the implanted antenna(s) 238 and does not interfere with the input received on the same lead to power the implant. In one embodiment the telemetry signal and powering signal are combined into one signal, where the RF telemetry signal is used to modulate the RF powering signal, and thus the implanted stimulator is powered directly by the received telemetry signal; separate subsystems in the stimulator harness the power contained in the signal and interpret the data content of the signal.

The RF switch 223 may be a multipurpose device such as a dual directional coupler, which passes the relatively high amplitude, extremely short duration RF pulse to the TX antenna 110 with minimal insertion loss while simultaneously providing two low-level outputs to feedback subsystem 212; one output delivers a forward power signal to the feedback subsystem 212, where the forward power signal is an attenuated version of the RF pulse sent to the TX antenna 110, and the other output delivers a reverse power signal to a different port of the feedback subsystem 212, where reverse power is an attenuated version of the reflected RF energy from the TX Antenna 110.

During the on-cycle time (when an RF signal is being transmitted to stimulator 114), the RF switch 223 is set to send the forward power signal to feedback subsystem. During the off-cycle time (when an RF signal is not being transmitted to the stimulator module 114), the RF switch 223 can change to a receiving mode in which the reflected RF energy and/or RF signals from the stimulator module 114 are received to be analyzed in the feedback subsystem 212.

The feedback subsystem 212 of the RF pulse generator module 106 may include reception circuitry to receive and extract telemetry or other feedback signals from the stimulator 114 and/or reflected RF energy from the signal sent by TX antenna 110. The feedback subsystem may include an amplifier 226, a filter 224, a demodulator 222, and an A/D converter 220.

The feedback subsystem 212 receives the forward power signal and converts this high-frequency AC signal to a DC level that can be sampled and sent to the controller subsystem 214. In this way the characteristics of the generated RF pulse can be compared to a reference signal within the controller subsystem 214. If a disparity (error) exists in any parameter, the controller subsystem 214 can adjust the output to the RF pulse generator 106. The nature of the adjustment can be, for example, proportional to the computed error. The controller subsystem 214 can incorporate additional inputs and limits on its adjustment scheme such as the signal amplitude of the reverse power and any predetermined maximum or minimum values for various pulse parameters.

The reverse power signal can be used to detect fault conditions in the RF-power delivery system. In an ideal condition, when TX antenna 110 has perfectly matched impedance to the tissue that it contacts, the electromagnetic waves generated from the RF pulse generator 106 pass unimpeded from the TX antenna 110 into the body tissue. However, in real-world applications a large degree of variability may exist in the body types of users, types of clothing worn, and positioning of the antenna 110 relative to the body surface. Since the impedance of the antenna 110 depends on the relative permittivity of the underlying tissue and any intervening materials, and also depends on the overall separation distance of the antenna from the skin, in any given application there can be an impedance mismatch at the interface of the TX antenna 110 with the body surface. When such a mismatch occurs, the electromagnetic waves sent from the RF pulse generator 106 are partially reflected at this interface, and this reflected energy propagates backward through the antenna feed.

The dual directional coupler RF switch 223 may prevent the reflected RF energy propagating back into the amplifier 226, and may attenuate this reflected RF signal and send the attenuated signal as the reverse power signal to the feedback subsystem 212. The feedback subsystem 212 can convert this high-frequency AC signal to a DC level that can be sampled and sent to the controller subsystem 214. The controller subsystem 214 can then calculate the ratio of the amplitude of the reverse power signal to the amplitude of the forward power signal. The ratio of the amplitude of reverse power signal to the amplitude level of forward power may indicate severity of the impedance mismatch.

In order to sense impedance mismatch conditions, the controller subsystem 214 can measure the reflected-power ratio in real time, and according to preset thresholds for this measurement, the controller subsystem 214 can modify the level of RF power generated by the RF pulse generator 106. For example, for a moderate degree of reflected power the course of action can be for the controller subsystem 214 to increase the amplitude of RF power sent to the TX antenna 110, as would be needed to compensate for slightly non-optimum but acceptable TX antenna coupling to the body. For higher ratios of reflected power, the course of action can be to prevent operation of the RF pulse generator 106 and set a fault code to indicate that the TX antenna 110 has little or no coupling with the body. This type of reflected-power fault condition can also be generated by a poor or broken connection to the TX antenna. In either case, it may be desirable to stop RF transmission when the reflected-power ratio is above a defined threshold, because internally reflected power can lead to unwanted heating of internal components, and this fault condition means the system cannot deliver sufficient power to the implanted wireless neural stimulator and thus cannot deliver therapy to the user.

The controller 242 of the stimulator 114 may transmit informational signals, such as a telemetry signal, through the antenna 238 to communicate with the RF pulse generator module 106 during its receive cycle. For example, the telemetry signal from the stimulator 114 may be coupled to the modulated signal on the dipole antenna(s) 238, during the on and off state of the transistor circuit to enable or disable a waveform that produces the corresponding RF bursts necessary to transmit to the external (or remotely implanted) pulse generator module 106. The antenna(s) 238 may be connected to electrodes 254 in contact with tissue to provide a return path for the transmitted signal. An A/D (not shown) converter can be used to transfer stored data to a serialized pattern that can be transmitted on the pulse modulated signal from the internal antenna(s) 238 of the neural stimulator.

A telemetry signal from the implanted wireless neural stimulator module 114 may include stimulus parameters such as the power or the amplitude of the current that is delivered to the tissue from the electrodes. The feedback signal can be transmitted to the RF pulse generator module 116 to indicate the strength of the stimulus at the nerve bundle by means of coupling the signal to the implanted RX antenna 238, which radiates the telemetry signal to the external (or remotely implanted) RF pulse generator module 106. The feedback signal can include either or both an analog and digital telemetry pulse modulated carrier signal. Data such as stimulation pulse parameters and measured characteristics of stimulator performance can be stored in an internal memory device within the implanted neural stimulator 114, and sent on the telemetry signal. The frequency of the carrier signal may be in the range of at 300 MHz to 8 GHz.

In the feedback subsystem 212, the telemetry signal can be down modulated using demodulator 222 and digitized by being processed through an analog to digital (A/D) converter 220. The digital telemetry signal may then be routed to a CPU 230 with embedded code, with the option to reprogram, to translate the signal into a corresponding current measurement in the tissue based on the amplitude of the received signal. The CPU 230 of the controller subsystem 214 can compare the reported stimulus parameters to those held in local memory 228 to verify the stimulator(s) 114 delivered the specified stimuli to tissue. For example, if the stimulator reports a lower current than was specified, the power level from the RF pulse generator module 106 can be increased so that the implanted neural stimulator 114 will have more available power for stimulation. The implanted neural stimulator 114 can generate telemetry data in real time, for example, at a rate of 8 kbits per second. All feedback data received from the implanted lead module 114 can be logged against time and sampled to be stored for retrieval to a remote monitoring system accessible by the health care professional for trending and statistical correlations.

The sequence of remotely programmable RF signals received by the internal antenna(s) 238 may be conditioned into waveforms that are controlled within the implantable stimulator 114 by the control subsystem 242 and routed to the appropriate electrodes 254 that are placed in proximity to the tissue to be stimulated. For instance, the RF signal transmitted from the RF pulse generator module 106 may be received by RX antenna 238 and processed by circuitry, such as waveform conditioning circuitry 240, within the implanted wireless neural stimulator module 114 to be converted into electrical pulses applied to the electrodes 254 through electrode interface 252. In some implementations, the implanted stimulator 114 contains between two to sixteen electrodes 254.

The waveform conditioning circuitry 240 may include a rectifier 244, which rectifies the signal received by the RX antenna 238. The rectified signal may be fed to the controller 242 for receiving encoded instructions from the RF pulse generator module 106. The rectifier signal may also be fed to a charge balance component 246 that is configured to create one or more electrical pulses based such that the one or more electrical pulses result in a substantially zero net charge at the one or more electrodes (that is, the pulses are charge balanced). The charge balanced pulses are passed through the current limiter 248 to the electrode interface 252, which applies the pulses to the electrodes 254 as appropriate.

The current limiter 248 insures the current level of the pulses applied to the electrodes 254 is not above a threshold current level. In some implementations, an amplitude (for example, current level, voltage level, or power level) of the received RF pulse directly determines the amplitude of the stimulus. In this case, it may be particularly beneficial to include current limiter 248 to prevent excessive current or charge being delivered through the electrodes, although current limiter 248 may be used in other implementations where this is not the case. Generally, for a given electrode having several square millimeters surface area, it is the charge per phase that should be limited for safety (where the charge delivered by a stimulus phase is the integral of the current). But, in some cases, the limit can instead be placed on the current, where the maximum current multiplied by the maximum possible pulse duration is less than or equal to the maximum safe charge. More generally, the limiter 248 acts as a charge limiter that limits a characteristic (for example, current or duration) of the electrical pulses so that the charge per phase remains below a threshold level (typically, a safe-charge limit).

In the event the implanted wireless neural stimulator 114 receives a “strong” pulse of RF power sufficient to generate a stimulus that would exceed the predetermined safe-charge limit, the current limiter 248 can automatically limit or “clip” the stimulus phase to maintain the total charge of the phase within the safety limit. The current limiter 248 may be a passive current limiting component that cuts the signal to the electrodes 254 once the safe current limit (the threshold current level) is reached. Alternatively, or additionally, the current limiter 248 may communicate with the electrode interface 252 to turn off all electrodes 254 to prevent tissue damaging current levels.

A clipping event may trigger a current limiter feedback control mode. The action of clipping may cause the controller to send a threshold power data signal to the pulse generator 106. The feedback subsystem 212 detects the threshold power signal and demodulates the signal into data that is communicated to the controller subsystem 214. The controller subsystem 214 algorithms may act on this current-limiting condition by specifically reducing the RF power generated by the RF pulse generator, or cutting the power completely. In this way, the pulse generator 106 can reduce the RF power delivered to the body if the implanted wireless neural stimulator 114 reports it is receiving excess RF power.

The controller 250 of the stimulator 205 may communicate with the electrode interface 252 to control various aspects of the electrode setup and pulses applied to the electrodes 254. The electrode interface 252 may act as a multiplex and control the polarity and switching of each of the electrodes 254. For instance, in some implementations, the wireless stimulator 106 has multiple electrodes 254 in contact with tissue, and for a given stimulus the RF pulse generator module 106 can arbitrarily assign one or more electrodes to 1) act as a stimulating electrode, 2) act as a return electrode, or 3) be inactive by communication of assignment sent wirelessly with the parameter instructions, which the controller 250 uses to set electrode interface 252 as appropriate. It may be physiologically advantageous to assign, for example, one or two electrodes as stimulating electrodes and to assign all remaining electrodes as return electrodes.

Also, in some implementations, for a given stimulus pulse, the controller 250 may control the electrode interface 252 to divide the current arbitrarily (or according to instructions from pulse generator module 106) among the designated stimulating electrodes. This control over electrode assignment and current control can be advantageous because in practice the electrodes 254 may be spatially distributed along various neural structures, and through strategic selection of the stimulating electrode location and the proportion of current specified for each location, the aggregate current distribution in tissue can be modified to selectively activate specific neural targets. This strategy of current steering can improve the therapeutic effect for the patient.

In another implementation, the time course of stimuli may be arbitrarily manipulated. A given stimulus waveform may be initiated at a time T_start and terminated at a time T_final, and this time course may be synchronized across all stimulating and return electrodes; further, the frequency of repetition of this stimulus cycle may be synchronous for all the electrodes. However, controller 250, on its own or in response to instructions from pulse generator 106, can control electrode interface 252 to designate one or more subsets of electrodes to deliver stimulus waveforms with non-synchronous start and stop times, and the frequency of repetition of each stimulus cycle can be arbitrarily and independently specified.

For example, a stimulator having eight electrodes may be configured to have a subset of five electrodes, called set A, and a subset of three electrodes, called set B. Set A might be configured to use two of its electrodes as stimulating electrodes, with the remainder being return electrodes. Set B might be configured to have just one stimulating electrode. The controller 250 could then specify that set A deliver a stimulus phase with 3 mA current for a duration of 200 us followed by a 400 us charge-balancing phase. This stimulus cycle could be specified to repeat at a rate of 60 cycles per second. Then, for set B, the controller 250 could specify a stimulus phase with 1 mA current for duration of 500 us followed by a 800 us charge-balancing phase. The repetition rate for the set-B stimulus cycle can be set independently of set A, say for example it could be specified at 25 cycles per second. Or, if the controller 250 was configured to match the repetition rate for set B to that of set A, for such a case the controller 250 can specify the relative start times of the stimulus cycles to be coincident in time or to be arbitrarily offset from one another by some delay interval.

In some implementations, the controller 250 can arbitrarily shape the stimulus waveform amplitude, and may do so in response to instructions from pulse generator 106. The stimulus phase may be delivered by a constant-current source or a constant-voltage source, and this type of control may generate characteristic waveforms that are static, e.g. a constant-current source generates a characteristic rectangular pulse in which the current waveform has a very steep rise, a constant amplitude for the duration of the stimulus, and then a very steep return to baseline. Alternatively, or additionally, the controller 250 can increase or decrease the level of current at any time during the stimulus phase and/or during the charge-balancing phase. Thus, in some implementations, the controller 250 can deliver arbitrarily shaped stimulus waveforms such as a triangular pulse, sinusoidal pulse, or Gaussian pulse for example. Similarly, the charge-balancing phase can be arbitrarily amplitude-shaped, and similarly a leading anodic pulse (prior to the stimulus phase) may also be amplitude-shaped.

As described above, the stimulator 114 may include a charge balancing component 246. Generally, for constant current stimulation pulses, pulses should be charge balanced by having the amount of cathodic current should equal the amount of anodic current, which is typically called biphasic stimulation. Charge density is the amount of current times the duration it is applied, and is typically expressed in the units uC/cm². In order to avoid the irreversible electrochemical reactions such as pH change, electrode dissolution as well as tissue destruction, no net charge should appear at the electrode-electrolyte interface, and it is generally acceptable to have a charge density less than 30 uC/cm². Biphasic stimulating current pulses ensure that no net charge appears at the electrode after each stimulation cycle and the electrochemical processes are balanced to prevent net dc currents. Neural stimulator 114 may be designed to ensure that the resulting stimulus waveform has a net zero charge. Charge balanced stimuli are thought to have minimal damaging effects on tissue by reducing or eliminating electrochemical reaction products created at the electrode-tissue interface.

A stimulus pulse may have a negative-voltage or current, called the cathodic phase of the waveform. Stimulating electrodes may have both cathodic and anodic phases at different times during the stimulus cycle. An electrode that delivers a negative current with sufficient amplitude to stimulate adjacent neural tissue is called a “stimulating electrode.” During the stimulus phase the stimulating electrode acts as a current sink. One or more additional electrodes act as a current source and these electrodes are called “return electrodes.” Return electrodes are placed elsewhere in the tissue at some distance from the stimulating electrodes. When a typical negative stimulus phase is delivered to tissue at the stimulating electrode, the return electrode has a positive stimulus phase. During the subsequent charge-balancing phase, the polarities of each electrode are reversed.

In some implementations, the charge balance component 246 uses a blocking capacitor(s) placed electrically in series with the stimulating electrodes and body tissue, between the point of stimulus generation within the stimulator circuitry and the point of stimulus delivery to tissue. In this manner, a resistor-capacitor (RC) network may be formed. In a multi-electrode stimulator, one charge-balance capacitor(s) may be used for each electrode or a centralized capacitor(s) may be used within the stimulator circuitry prior to the point of electrode selection. The RC network can block direct current (DC), however it can also prevent low-frequency alternating current (AC) from passing to the tissue. The frequency below which the series RC network essentially blocks signals is commonly referred to as the cutoff frequency, and in one embodiment the design of the stimulator system may ensure the cutoff frequency is not above the fundamental frequency of the stimulus waveform. In this embodiment of the present invention, the wireless stimulator may have a charge-balance capacitor with a value chosen according to the measured series resistance of the electrodes and the tissue environment in which the stimulator is implanted. By selecting a specific capacitance value the cutoff frequency of the RC network in this embodiment is at or below the fundamental frequency of the stimulus pulse.

In other implementations, the cutoff frequency may be chosen to be at or above the fundamental frequency of the stimulus, and in this scenario the stimulus waveform created prior to the charge-balance capacitor, called the drive waveform, may be designed to be non-stationary, where the envelope of the drive waveform is varied during the duration of the drive pulse. For example, in one embodiment, the initial amplitude of the drive waveform is set at an initial amplitude Vi, and the amplitude is increased during the duration of the pulse until it reaches a final value k*Vi. By changing the amplitude of the drive waveform over time, the shape of the stimulus waveform passed through the charge-balance capacitor is also modified. The shape of the stimulus waveform may be modified in this fashion to create a physiologically advantageous stimulus.

In some implementations, the wireless neural stimulator module 114 may create a drive-waveform envelope that follows the envelope of the RF pulse received by the receiving dipole antenna(s) 238. In this case, the RF pulse generator module 106 can directly control the envelope of the drive waveform within the wireless neural stimulator 114, and thus no energy storage may be required inside the stimulator itself. In this implementation, the stimulator circuitry may modify the envelope of the drive waveform or may pass it directly to the charge-balance capacitor and/or electrode-selection stage.

In some implementations, the implanted neural stimulator 114 may deliver a single-phase drive waveform to the charge balance capacitor or it may deliver multiphase drive waveforms. In the case of a single-phase drive waveform, for example, a negative-going rectangular pulse, this pulse comprises the physiological stimulus phase, and the charge-balance capacitor is polarized (charged) during this phase. After the drive pulse is completed, the charge balancing function is performed solely by the passive discharge of the charge-balance capacitor, where is dissipates its charge through the tissue in an opposite polarity relative to the preceding stimulus. In one implementation, a resistor within the stimulator facilitates the discharge of the charge-balance capacitor. In some implementations, using a passive discharge phase, the capacitor may allow virtually complete discharge prior to the onset of the subsequent stimulus pulse.

In the case of multiphase drive waveforms the wireless stimulator may perform internal switching to pass negative-going or positive-going pulses (phases) to the charge-balance capacitor. These pulses may be delivered in any sequence and with varying amplitudes and waveform shapes to achieve a desired physiological effect. For example, the stimulus phase may be followed by an actively driven charge-balancing phase, and/or the stimulus phase may be preceded by an opposite phase. Preceding the stimulus with an opposite-polarity phase, for example, can have the advantage of reducing the amplitude of the stimulus phase required to excite tissue.

In some implementations, the amplitude and timing of stimulus and charge-balancing phases is controlled by the amplitude and timing of RF pulses from the RF pulse generator module 106, and in others this control may be administered internally by circuitry onboard the wireless stimulator 114, such as controller 250. In the case of onboard control, the amplitude and timing may be specified or modified by data commands delivered from the pulse generator module 106.

FIG. 3 is a flowchart showing an example of an operation of the neural stimulator system. In block 302, the wireless neural stimulator 114 is implanted in proximity to nerve bundles and is coupled to the electric field produced by the TX antenna 110. That is, the pulse generator module 106 and the TX antenna 110 are positioned in such a way (for example, in proximity to the patient) that the TX antenna 110 is electrically radiatively coupled with the implanted RX antenna 238 of the neural stimulator 114. In certain implementations, both the antenna 110 and the RF pulse generator 106 are located subcutaneously. In other implementations, the antenna 110 and the RF pulse generator 106 are located external to the patient's body. In this case, the TX antenna 110 may be coupled directly to the patient's skin.

Energy from the RF pulse generator is radiated to the implanted wireless neural stimulator 114 from the antenna 110 through tissue, as shown in block 304. The energy radiated may be controlled by the Patient/Clinician Parameter inputs in block 301. In some instances, the parameter settings can be adjusted in an open loop fashion by the patient or clinician, who would adjust the parameter inputs in block 301 to the system.

The wireless implanted stimulator 114 uses the received energy to generate electrical pulses to be applied to the neural tissue through the electrodes 238. For instance, the stimulator 114 may contain circuitry that rectifies the received RF energy and conditions the waveform to charge balance the energy delivered to the electrodes to stimulate the targeted nerves or tissues, as shown in block 306. The implanted stimulator 114 communicates with the pulse generator 106 by using antenna 238 to send a telemetry signal, as shown in block 308. The telemetry signal may contain information about parameters of the electrical pulses applied to the electrodes, such as the impedance of the electrodes, whether the safe current limit has been reached, or the amplitude of the current that is presented to the tissue from the electrodes.

In block 310, the RF pulse generator 106 detects amplifies, filters and modulates the received telemetry signal using amplifier 226, filter 224, and demodulator 222, respectively. The A/D converter 230 then digitizes the resulting analog signal, as shown in 312. The digital telemetry signal is routed to CPU 230, which determines whether the parameters of the signal sent to the stimulator 114 need to be adjusted based on the digital telemetry signal. For instance, in block 314, the CPU 230 compares the information of the digital signal to a look-up table, which may indicate an appropriate change in stimulation parameters. The indicated change may be, for example, a change in the current level of the pulses applied to the electrodes. As a result, the CPU may change the output power of the signal sent to stimulator 114 so as to adjust the current applied by the electrodes 254, as shown in block 316.

Thus, for instance, the CPU 230 may adjust parameters of the signal sent to the stimulator 114 every cycle to match the desired current amplitude setting programmed by the patient, as shown in block 318. The status of the stimulator system may be sampled in real time at a rate of 8 kbits per second of telemetry data. All feedback data received from the stimulator 114 can be maintained against time and sampled per minute to be stored for download or upload to a remote monitoring system accessible by the health care professional for trending and statistical correlations in block 318. If operated in an open loop fashion, the stimulator system operation may be reduced to just the functional elements shown in blocks 302, 304, 306, and 308, and the patient uses their judgment to adjust parameter settings rather than the closed looped feedback from the implanted device.

FIG. 4 depicts a flow chart showing an example of an operation of the system when the current level at the electrodes 254 is above a threshold limit. In certain instances, the implanted wireless neural stimulator 114 may receive an input power signal with a current level above an established safe current limit, as shown in block 402. For instance, the current limiter 248 may determine the current is above an established tissue-safe limit of amperes, as shown in block 404. If the current limiter senses that the current is above the threshold, it may stop the high-power signal from damaging surrounding tissue in contact with the electrodes as shown in block 406, the operations of which are as described above in association with FIG. 2.

A capacitor may store excess power, as shown in block 408. When the current limiter senses the current is above the threshold, the controller 250 may use the excess power available to transmit a small 2-bit data burst back to the RF pulse generator 106, as shown in block 410. The 2-bit data burst may be transmitted through the implanted wireless neural stimulator's antenna(s) 238 during the RF pulse generator's receive cycle, as shown in block 412. The RF pulse generator antenna 110 may receive the 2-bit data burst during its receive cycle, as shown in block 414, at a rate of 8 kbps, and may relay the data burst back to the RF pulse generator's feedback subsystem 212 which is monitoring all reverse power, as shown in block 416. The CPU 230 may analyze signals from feedback subsystem 202, as shown in block 418 and if there is no data burst present, no changes may be made to the stimulation parameters, as shown in block 420. If the data burst is present in the analysis, the CPU 230 can cut all transmission power for one cycle, as shown in block 422.

If the data burst continues, the RF pulse generator 106 may push a “proximity power danger” notification to the application on the programmer module 102, as shown in block 424. This proximity danger notification occurs because the RF pulse generator has ceased its transmission of power. This notification means an unauthorized form of energy is powering the implant above safe levels. The application may alert the user of the danger and that the user should leave the immediate area to resume neural modulation therapy, as shown in block 426. If after one cycle the data burst has stopped, the RF pulse generator 106 may slowly ramp up the transmission power in increments, for example from 5% to 75% of previous current amplitude levels, as shown in block 428. The user can then manually adjust current amplitude level to go higher at the user's own risk. During the ramp up, the RF pulse generator 106 may notify the application of its progress and the application may notify the user that there was an unsafe power level and the system is ramping back up, as shown in block 430.

FIG. 5 is a diagram showing examples of signals that may be used to detect an impedance mismatch. As described above, a forward power signal and a reverse power signal may be used to detect an impedance mismatch. For instance, a RF pulse 502 generated by the RF pulse generator may pass through a device such as a dual directional coupler to the TX antenna 110. The TX antenna 110 then radiates the RF signal into the body, where the energy is received by the implanted wireless neural stimulator 114 and converted into a tissue-stimulating pulse. The coupler passes an attenuated version of this RF signal, forward power 510, to feedback subsystem 212. The feedback subsystem 212 demodulates the AC signal and computes the amplitude of the forward RF power, and this data is passed to controller subsystem 214. Similarly the dual directional coupler (or similar component) also receives RF energy reflected back from the TX antenna 110 and passes an attenuated version of this RF signal, reverse power 512, to feedback subsystem 212. The feedback subsystem 212 demodulates the AC signal and computes the amplitude of the reflected RF power, and this data is passed to controller subsystem 214.

In the optimal case, when the TX antenna 110 may be perfectly impedance-matched to the body so that the RF energy passes unimpeded across the interface of the TX antenna 110 to the body, and no RF energy is reflected at the interface. Thus, in this optimal case, the reverse power 512 may have close to zero amplitude as shown by signal 504, and the ratio of reverse power 512 to forward power 510 is zero. In this circumstance, no error condition exists, and the controller 214 sets a system message that operation is optimal.

In practice, the impedance match of the TX antenna 204 to the body may not be optimal, and some energy of the RF pulse 502 is reflected from the interface of the TX antenna 110 and the body. This can occur for example if the TX antenna 110 is held somewhat away from the skin by a piece of clothing. This non-optimal antenna coupling causes a small portion of the forward RF energy to be reflected at the interface, and this is depicted as signal 506. In this case, the ratio of reverse power 512 to forward power 510 is small, but a small ratio implies that most of the RF energy is still radiated from the TX antenna 110, so this condition is acceptable within the control algorithm. This determination of acceptable reflection ratio may be made within controller subsystem 214 based upon a programmed threshold, and the controller subsystem 214 may generate a low-priority alert to be sent to the user interface. In addition, the controller subsystem 214 sensing the condition of a small reflection ratio, may moderately increase the amplitude of the RF pulse 502 to compensate for the moderate loss of forward energy transfer to the implanted wireless neural stimulator 114.

During daily operational use, the TX antenna 110 might be accidentally removed from the body entirely, in which case the TX antenna will have very poor coupling to the body (if any). In this or other circumstances, a relatively high proportion of the RF pulse energy is reflected as signal 508 from the TX antenna 110 and fed backward into the RF-powering system. Similarly, this phenomenon can occur if the connection to the TX antenna is physically broken, in which case virtually 100% of the RF energy is reflected backward from the point of the break. In such cases, the ratio of reverse power 512 to forward power 510 is very high, and the controller subsystem 214 will determine the ratio has exceeded the threshold of acceptance. In this case, the controller subsystem 214 may prevent any further RF pulses from being generated. The shutdown of the RF pulse generator module 106 may be reported to the user interface to inform the user that stimulation therapy cannot be delivered.

FIG. 6 is a diagram showing examples of signals that may be employed during operation of the neural stimulator system. According to some implementations, the amplitude of the RF pulse 602 received by the implanted wireless neural stimulator 114 can directly control the amplitude of the stimulus 630 delivered to tissue. The duration of the RF pulse 608 corresponds to the specified pulse width of the stimulus 630. During normal operation the RF pulse generator module 106 sends an RF pulse waveform 602 via TX antenna 110 into the body, and RF pulse waveform 608 may represent the corresponding RF pulse received by implanted wireless neural stimulator 114. In this instance the received power has an amplitude suitable for generating a safe stimulus pulse 630. The stimulus pulse 630 is below the safety threshold 626, and no error condition exists. In another example, the attenuation between the TX antenna 110 and the implanted wireless neural stimulator 114 has been unexpectedly reduced, for example due to the user repositioning the TX antenna 110. This reduced attenuation can lead to increased amplitude in the RF pulse waveform 612 being received at the neural stimulator 114. Although the RF pulse 602 is generated with the same amplitude as before, the improved RF coupling between the TX antenna 110 and the implanted wireless neural stimulator 114 can cause the received RF pulse 612 to be larger in amplitude. Implanted wireless neural stimulator 114 in this situation may generate a larger stimulus 632 in response to the increase in received RF pulse 612. However, in this example, the received power 612 is capable of generating a stimulus 632 that exceeds the prudent safety limit for tissue. In this situation, the current limiter feedback control mode can operate to clip the waveform of the stimulus pulse 632 such that the stimulus delivered is held within the predetermined safety limit 626. The clipping event 628 may be communicated through the feedback subsystem 212 as described above, and subsequently controller subsystem 214 can reduce the amplitude specified for the RF pulse. As a result, the subsequent RF pulse 604 is reduced in amplitude, and correspondingly the amplitude of the received RF pulse 616 is reduced to a suitable level (non-clipping level). In this fashion, the current limiter feedback control mode may operate to reduce the RF power delivered to the body if the implanted wireless neural stimulator 114 receives excess RF power.

In another example, the RF pulse waveform 606 depicts a higher amplitude RF pulse generated as a result of user input to the user interface. In this circumstance, the RF pulse 620 received by the implanted wireless neural stimulator 14 is increased in amplitude, and similarly current limiter feedback mode operates to prevent stimulus 636 from exceeding safety limit 626. Once again, this clipping event 628 may be communicated through the feedback subsystem 212, and subsequently controller subsystem 214 may reduce the amplitude of the RF pulse, thus overriding the user input. The reduced RF pulse 604 can produce correspondingly smaller amplitudes of the received waveforms 616, and clipping of the stimulus current may no longer be required to keep the current within the safety limit. In this fashion, the current limiter feedback may reduce the RF power delivered to the body if the implanted wireless neural stimulator 114 reports it is receiving excess RF power.

FIG. 7 is a flow chart showing a process for the user to control the implantable wireless neural stimulator through the programmer in an open loop feedback system. In one implementation of the system, the user has a wireless neural stimulator implanted in their body, the RF pulse generator 106 sends the stimulating pulse power wirelessly to the stimulator 114, and an application on the programmer module 102 (for example, a smart device) is communicating with the RF pulse generator 106. In this implementation, if a user wants to observe the current status of the functioning pulse generator, as shown in block 702, the user may open the application, as shown in block 704. The application can use Bluetooth protocols built into the smart device to interrogate the pulse generator, as shown in block 706. The RF pulse generator 106 may authenticate the identity of the smart device and serialized patient assigned secure iteration of the application, as shown in block 708. The authentication process may utilize a unique key to the patient specific RF pulse generator serial number. The application can be customized with the patient specific unique key through the Manufacturer Representative who has programmed the initial patient settings for the stimulation system, as shown in block 720. If the RF pulse generator rejects the authentication it may inform the application that the code is invalid, as shown in block 718 and needs the authentication provided by the authorized individual with security clearance from the device manufacturer, known as the “Manufacturer's Representative,” as shown in block 722. In an implementation, only the Manufacturer's Representative can have access to the security code needed to change the application's stored RF pulse generator unique ID. If the RF pulse generator authentication system passes, the pulse generator module 106 sends back all of the data that has been logged since the last sync, as shown in block 710. The application may then register the most current information and transmit the information to a 3rd party in a secure fashion, as shown in 712. The application may maintain a database that logs all system diagnostic results and values, the changes in settings by the user and the feedback system, and the global runtime history, as shown in block 714. The application may then display relevant data to the user, as shown in block 716; including the battery capacity, current program parameter, running time, pulse width, frequency, amplitude, and the status of the feedback system.

FIG. 8 is another example flow chart of a process for the user to control the wireless stimulator with limitations on the lower and upper limits of current amplitude. The user wants to change the amplitude of the stimulation signal, as shown in block 802. The user may open the application, as show in block 704 and the application may go through the process described in FIG. 7 to communicate with the RF pulse generator, authenticate successfully, and display the current status to the user, as shown in block 804. The application displays the stimulation amplitude as the most prevalent changeable interface option and displays two arrows with which the user can adjust the current amplitude. The user may make a decision based on their need for more or less stimulation in accordance with their pain levels, as shown in block 806. If the user chooses to increase the current amplitude, the user may press the up arrow on the application screen, as shown in block 808. The application can include safety maximum limiting algorithms, so if a request to increase current amplitude is recognized by the application as exceeding the preset safety maximum, as shown in block 810, then the application will display an error message, as shown in block 812 and will not communicate with the RF pulse generator module 106. If the user presses the up arrow, as shown in block 808 and the current amplitude request does not exceed the current amplitude maximum allowable value, then the application will send instructions to the RF pulse generator module 106 to increase amplitude, as shown in block 814. The RF pulse generator module 106 may then attempt to increase the current amplitude of stimulation, as shown in block 816. If the RF pulse generator is successful at increasing the current amplitude, the RF pulse generator module 106 may perform a short vibration to physically confirm with the user that the amplitude is increased, as shown in block 818. The RF pulse generator module 106 can also send back confirmation of increased amplitude to the application, as shown in block 820, and then the application may display the updated current amplitude level, as shown in block 822.

If the user decides to decrease the current amplitude level in block 806, the user can press the down arrow on the application, as shown in block 828. If the current amplitude level is already at zero, the application recognizes that the current amplitude cannot be decreased any further, as shown in block 830 and displays an error message to the user without communicating any data to the RF pulse generator, as shown in block 832. If the current amplitude level is not at zero, the application can send instructions to the RF pulse generator module 106 to decrease current amplitude level accordingly, as shown in block 834. The RF pulse generator may then attempt to decrease current amplitude level of stimulation RF pulse generator module 106 and, if successful, the RF pulse generator module 106 may perform a short vibration to physically confirm to the user that the current amplitude level has been decreased, as shown in block 842. The RF pulse generator module 106 can send back confirmation of the decreased current amplitude level to the application, as shown in block 838. The application then may display the updated current amplitude level, as indicated by block 840. If the current amplitude level decrease or increase fails, the RF pulse generator module 106 can perform a series of short vibrations to alert user, and send an error message to the application, as shown in block 824. The application receives the error and may display the data for the user's benefit, as shown in block 826.

FIG. 9 is yet another example flow chart of a process for the user to control the wireless neural stimulator 114 through preprogrammed parameter settings. The user wants to change the parameter program, as indicated by block 902. When the user is implanted with a wireless neural stimulator or when the user visits the doctor, the Manufacturer's Representative may determine and provide the patient/user RF pulse generator with preset programs that have different stimulation parameters that will be used to treat the user. The user will then able to switch between the various parameter programs as needed. The user can open the application on their smart device, as indicated by block 704, which first follows the process described in FIG. 7, communicating with the RF pulse generator module 106, authenticating successfully, and displaying the current status of the RF pulse generator module 106, including the current program parameter settings, as indicated by block 812. In this implementation, through the user interface of the application, the user can select the program that they wish to use, as shown by block 904. The application may then access a library of pre-programmed parameters that have been approved by the Manufacturer's Representative for the user to interchange between as desired and in accordance with the management of their indication, as indicated by block 906. A table can be displayed to the user, as shown in block 908 and each row displays a program's codename and lists its basic parameter settings, as shown in block 910, which includes but is not limited to: pulse width, frequency, cycle timing, pulse shape, duration, feedback sensitivity, as shown in block 912. The user may then select the row containing the desired parameter preset program to be used, as shown in block 912. The application can send instructions to the RF pulse generator module 106 to change the parameter settings, as shown in block 916. The RF pulse generator module 106 may attempt to change the parameter settings 154. If the parameter settings are successfully changed, the RF pulse generator module 106 can perform a unique vibration pattern to physically confirm with the user that the parameter settings were changed, as shown in block 920. Also, the RF pulse generator module 106 can send back confirmation to the application that the parameter change has been successful, as shown in block 922, and the application may display the updated current program, as shown in block 924. If the parameter program change has failed, the RF pulse generator module 106 may perform a series of short vibrations to alert the user, and send an error message to the application, as shown in block 926, which receives the error and may display to the user, as shown in block 928.

FIG. 10 is still another example flow chart of a process for a low battery state for the RF pulse generator module 106. In this implementation, the RF pulse generator module's remaining battery power level is recognized as low, as shown in block 1002. The RF pulse generator module 106 regularly interrogates the power supply battery subsystem 210 about the current power and the RF pulse generator microprocessor asks the battery if its remaining power is below threshold, as shown in block 1004. If the battery's remaining power is above the threshold, the RF pulse generator module 106 may store the current battery status to be sent to the application during the next sync, as shown in block 1006. If the battery's remaining power is below threshold the RF pulse generator module 106 may push a low-battery notification to the application, as shown in block 1008. The RF pulse generator module 106 may always perform one sequence of short vibrations to alert the user of an issue and send the application a notification, as shown in block 1010. If there continues to be no confirmation of the application receiving the notification then the RF pulse generator can continue to perform short vibration pulses to notify user, as shown in block 1010. If the application successfully receives the notification, it may display the notification and may need user acknowledgement, as shown in block 1012. If, for example, one minute passes without the notification message on the application being dismissed the application informs the RF pulse generator module 106 about lack of human acknowledgement, as shown in block 1014, and the RF pulse generator module 106 may begin to perform the vibration pulses to notify the user, as shown in block 1010. If the user dismisses the notification, the application may display a passive notification to switch the battery, as shown in block 1016. If a predetermined amount of time passes, such as five minutes for example, without the battery being switched, the application can inform the RF pulse generator module 106 of the lack of human acknowledgement, as shown in block 1014 and the RF pulse generator module 106 may perform vibrations, as shown in block 1010. If the RF pulse generator module battery is switched, the RF pulse generator module 106 reboots and interrogates the battery to assess power remaining, as shown in block 1618. If the battery's power remaining is below threshold, the cycle may begin again with the RF pulse generator module 106 pushing a notification to the application, as shown in block 1008. If the battery's power remaining is above threshold the RF pulse generator module 106 may push a successful battery-change notification to the application, as shown in block 1620. The application may then communicate with the RF pulse generator module 106 and displays current system status, as shown in block 1022.

FIG. 11 is yet another example flow chart of a process for a Manufacturer's Representative to program the implanted wireless neural stimulator. In this implementation, a user wants the Manufacturer's Representative to set individual parameter programs from a remote location different than where the user is, for the user to use as needed, as shown in block 1102. The Manufacturer's Representative can gain access to the user's set parameter programs through a secure web based service. The Manufacturer's Representative can securely log into the manufacturer's web service on a device connected to the Internet, as shown in block 1104. If the Manufacturer's Representative is registering the user for the first time in their care they enter in the patient's basic information, the RF pulse generator's unique ID and the programming application's unique ID, as shown in block 1106. Once the Manufacturer's Representative's new or old user is already registered, the Manufacturer's Representative accesses the specific user's profile, as shown in block 1108. The Manufacturer's Representative is able to view the current allotted list of parameter programs for the specific user, as shown in block 1110. This list may contain previous active and retired parameter preset programs, as shown in block 1112. The Manufacturer's Representative is able to activate/deactivate preset parameter programs by checking the box next to the appropriate row in the table displayed, as shown in block 1114. The Manufacturer's Representative may then submit and save the allotted new preset parameter programs, as shown in block 1116. The user's programmer application may receive the new preset parameter programs at the next sync with the manufacturer's database.

FIG. 12 is a circuit diagram showing an example of a wireless neural stimulator, such as stimulator 114. This example contains paired electrodes, comprising cathode electrode(s) 1208 and anode electrode(s) 1210, as shown. When energized, the charged electrodes create a volume conduction field of current density within the tissue. In this implementation, the wireless energy is received through a dipole antenna(s) 238. At least four diodes are connected together to form a full wave bridge rectifier 1202 attached to the dipole antenna(s) 238. Each diode, up to 100 micrometers in length, uses a junction potential to prevent the flow of negative electrical current, from cathode to anode, from passing through the device when said current does not exceed the reverse threshold. For neural stimulation via wireless power, transmitted through tissue, the natural inefficiency of the lossy material may lead to a low threshold voltage. In this implementation, a zero biased diode rectifier results in a low output impedance for the device. A resistor 1204 and a smoothing capacitor 1206 are placed across the output nodes of the bridge rectifier to discharge the electrodes to the ground of the bridge anode. The rectification bridge 1202 includes two branches of diode pairs connecting an anode-to-anode and then cathode to cathode. The electrodes 1208 and 1210 are connected to the output of the charge balancing circuit 246.

FIG. 13 is a circuit diagram of another example of a wireless neural stimulator, such as stimulator 114. The example shown in FIG. 13 includes multiple electrode control and may employ full closed loop control. The stimulator includes an electrode array 254 in which the polarity of the electrodes can be assigned as cathodic or anodic, and for which the electrodes can be alternatively not powered with any energy. When energized, the charged electrodes create a volume conduction field of current density within the tissue. In this implementation, the wireless energy is received by the device through the dipole antenna(s) 238. The electrode array 254 is controlled through an on-board controller circuit 242 that sends the appropriate bit information to the electrode interface 252 in order to set the polarity of each electrode in the array, as well as power to each individual electrode. The lack of power to a specific electrode would set that electrode in a functional OFF position. In another implementation (not shown), the amount of current sent to each electrode is also controlled through the controller 242. The controller current, polarity and power state parameter data, shown as the controller output, is be sent back to the antenna(s) 238 for telemetry transmission back to the pulse generator module 106. The controller 242 also includes the functionality of current monitoring and sets a bit register counter so that the status of total current drawn can be sent back to the pulse generator module 106.

At least four diodes can be connected together to form a full wave bridge rectifier 302 attached to the dipole antenna(s) 238. Each diode, up to 100 micrometers in length, uses a junction potential to prevent the flow of negative electrical current, from cathode to anode, from passing through the device when said current does not exceed the reverse threshold. For neural stimulation via wireless power, transmitted through tissue, the natural inefficiency of the lossy material may lead to a low threshold voltage. In this implementation, a zero biased diode rectifier results in a low output impedance for the device. A resistor 1204 and a smoothing capacitor 1206 are placed across the output nodes of the bridge rectifier to discharge the electrodes to the ground of the bridge anode. The rectification bridge 1202 may include two branches of diode pairs connecting an anode-to-anode and then cathode to cathode. The electrode polarity outputs, both cathode 1208 and anode 1210 are connected to the outputs formed by the bridge connection. Charge balancing circuitry 246 and current limiting circuitry 248 are placed in series with the outputs.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims. 

What is claimed is:
 1. A system comprising: an external controller module comprising: a first dipole antenna configured to: transmit an input signal containing electrical energy to a second dipole antenna through electrical radiative coupling to wirelessly power a passive implantable neural stimulator that houses the second dipole antenna; and one or more circuits configured to: generate the input signal and send the input signal to the first antenna for transmission to the second antenna on the passive implantable neural stimulator such that the passive implantable neural stimulator, solely using the electrical energy contained in the input signal, (i) creates and applies one or more electrical pulses suitable for stimulation of neural tissue, and (ii) monitors the stimulation to generate a current limit feedback signal when a clipping event has occurred in which a characteristic of the one or more electrical pulses had been limited so as to prevent the one or more electrical pulses from exceeding a safety threshold; and in response to the current feedback signal being received, attenuate the input signal so that clipping of the one or more electrical pulses is no longer required to keep the current within the safety threshold.
 2. The system of claim 1, wherein one or more circuits configured to: generate the input signal and send the input signal to the first antenna for transmission to the second antenna on the passive implantable neural stimulator such that the passive implantable neural stimulator, solely using the electrical energy contained in the input signal, measures one or more parameters of the electrical pulses including an amplitude of the one or more electrical pulses as applied to the neural tissue; and adjust a power of the input signal based on the amplitude of the one or more electrical pulses.
 3. The system of claim 1 wherein the one or more circuits are configured to: obtain a forward power signal that is reflective of an amplitude of a radio-frequency (RF) signal sent to the first antenna; obtain a reverse power signal that is reflective of an amplitude of a reflected portion of the RF signal sent to the first antenna; determine a mismatch value indicative of a magnitude of an impedance mismatch based on the forward power signal and the reverse power signal; and adjust the parameters of the input signal based on the mismatch value.
 4. The system of claim 1 further comprising the implantable neural stimulator, the implantable neural stimulator including: one or more electrodes configured to apply the one or more electrical pulses to neural tissue; one or more circuits configured to: create the one or more electrical pulses; supply the one or more electrical pulses to the one or more electrodes such that the one or more electrodes apply the one or more electrical pulses to neural tissue; generate the current limit feedback signal; and send the current limit feedback signal to the second antenna such that the second antenna transmits the current limit feedback signal to the first antenna through electrical radiative coupling.
 5. The system of claim 4, wherein the input signal also contains information encoding stimulus parameters for the one or more electrical pulses and the implantable neural stimulator is configured to create the one or more electrical pulses based on the information encoding stimulus parameters.
 6. The system of claim 4 wherein the one or more circuits of the implantable neural stimulator are configured such that a level of the input signal directly determines an amplitude of the one or more electrical pulses applied to the neural tissue by the one or more electrodes.
 7. The system of claim 6 wherein: the implantable neural stimulator includes a plurality of electrodes; the one or more circuits of the external controller module are configured to: generate a control signal that designates which electrodes act as stimulating electrodes, which electrodes act as return electrodes that are located further away from the neural tissue being stimulated than the stimulating electrodes, and which electrodes are inactive; and send the control signal to the first antenna such that the first antenna transmits the control signal to the second antenna through electrical radiative coupling; and the one or more circuits of the implantable neural stimulator are configured to selectively designate each of the electrodes to act as a stimulating electrode, act as a return electrode, or be inactive based on the control signal.
 8. The system of claim 6 wherein the implantable neural stimulator does not include an internal power source.
 9. The system of claim 6 wherein the one or more circuits of the implantable neural stimulator include only passive components.
 10. The system of claim 4 wherein the one or more circuits of the implantable neural stimulator are configured to: limit a characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes so that the charge per phase resulting from the one or more electrical pulses remain below the threshold level ; generate the limit feedback signal when the charge per phase resulting from the one or more electrical pulses would have exceeded the threshold level if the one or more circuits had not limited the characteristic of the one or more electrical pulses applied to the neural tissue by the one or more electrodes so that the charge per phase resulting from the one or more electrical pulses remained below the threshold level; and send the limit feedback signal to the second antenna such that the second antenna transmits the limit feedback signal to the first antenna through electrical radiative coupling.
 11. The system of claim 4 wherein the one or more circuits are configured to create the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge.
 12. The system of claim 11 wherein, to create the one or more electrical pulses such that the one or more electrical pulses result in a substantially zero net charge, the one or more circuits of the implantable neural stimulator include at least one capacitor in series with the one or more electrodes.
 13. The system of claim 4 wherein the one or more circuits of the implantable neural stimulator include: a waveform conditioning component to create the one or more electrical pulses suitable for stimulation of neural tissue using electrical energy contained in the input signal; an electrode interface connected to the waveform conditioning circuit, the electrode interface being configured to receive the one or more electrical pulses from the waveform conditioning circuit and supply the one or more electrical pulses to the one or more electrodes; and a controller connected to the electrode interface, the controller being configured to generate the stimulus feedback signal and send the stimulus feedback signal to the second antenna.
 14. The system of claim 13 wherein the waveform conditioning component includes: a rectifier connected to the second antenna, the rectifier configured to receive the input signal from the second antenna and generate a rectified electrical waveform based on the input signal; a charge balance component configured to create the one or more electrical pulses based on the rectified electrical waveform such that the one or more electrical pulses result in a substantially zero net charge at the one or more electrodes; and a charge limiter configured to limit a characteristic of the one or more electrical pulses so that a charge per phase resulting from the one or more electrical pulses remains below a threshold level, wherein the limited electrical pulses are sent to the electrode interface through the charge limiter. 